Clearances. It is very important that each member of a struc ture fit together well in the field; and it is equally important that the draftsman should so detail his work that the various parts of any particular member should, without further cutting than the first, fit together. Also the rivets should be so spaced and placed that they can be driven.
The rivet clearances have been mentioned under "Rivets and Rivet Spacing" and will not be taken up here. It is sufficient to say that on the rivet clearances is where the novice makes the most of his mistakes.
Where the distance between the outer faces of several mem bers placed together is to be com puted, it is necessary, on account of the liability of plates to exceed their nominal thicknesses, and rivet heads their nominal height, to make certain allowances. The usual practice is: (1) Between eye (or loop) bars allow is inch.
(2) Between an eye( or loop) bar and a built-up member inch.
(3) Between two built-up members inch.
For example, suppose it was required to compute the distance out to out of the members shown in Fig. 47. The clearance would be as indicated, and the distance D would be: This value would be the grip of the pin which was used at this joint. The 0.4 inch and 0.2S inch in the above are the thicknesses of the channel webs, and the inch is the height of a s inch rivet head.
In the use of eye bars, it is essential to see that their heads as well as their bodies clear. In order to determine the dimension of a section for the necessary clearance, the size of the head must be ascertained. This is best done by drawing up the head to a large scale. The method of procedure is as follows: (1) Draw the circle representing the pinhole; (2) for the width of eye bar under con sideration, subtract the radius of the largest pinhole in Cambria for that bar from the radius of the given head and add the result to one-half the pinhole diameter in your particular case, thus giving you R, Fig. 4G; (3) with the radius R describe a full circle; (4) with the center of the pin as a center and a radius equal to 2 R describe a couple of arcs 1, 1; (5) parallel to the bar and at a distance 1; R from it, draw two lines, 2, 2, intersecting the arcs 1, 1; and (6) with these intersections as centers and a radius equal to 1; 1? describe the small arcs completing the head, see Fig. 46.
No material should be closer to the edge of the eye-bar head than 2 inch. This clearance should always be given, see Fig. 48, In case the head is on the interior of a channel or so as to come near the fillet of an angle, the 1 inch must be measured from the curve of the fillet. This inch does not apply to the body of the bar, the clearance there being inch in accordance with what follows.
Wherever several pieces of metal are riveted to the same side of a plate or other member and could, theoretically, come close against each other, 1-inch clearance is allowed for each case where the ends are not planed. This allows for the slight variations in length liable to occur when the surfaces are sheared. The members will then be sufficiently close together for all practical purposes. In order that no errors occur, the joint should be drawn up on a separate sheet to a scale of at least 11 or 2 inches to the foot in case the pieces meet at an angle. In case the pieces meet at right angles, the distances may be computed. Fig. 49 gives a few of the most common eases.
As in the case of Fig. 49c and 49d the clear ances at one end will be inch and at the other end may be more, and should be, in order that the distances /1 and /2 shall he the same. (The distance from the first rivet to the end of the angle is usually 11 or 11, generally the latter.) It must not be understood that the clearance is exactly inch; it must be at least I inch, and may be more, up to s inch or i inch in order that the distance from the rivet to some other point or rivet may be in an even inch or -1 inch.
When Fbeams or channels are placed as mentioned above, .1-inch clearance or more instead of the 1-inch is required, one of the most common eases where such clearance is required being shown in Fig. 50. For other clearances in beams see "The Detailing of Beams," page 72.
Wherever bolts, rods, upsets, or rolled bars pass through a hole or slot, the aperture should be s inch greater in diameter or 4 inch greater in dimensions in ease there is a slot. The above is in case the material is rolled steel or iron. In case of a casting, 4 inch should he added to the dimensions of the member which is to pass through the opening.